Bee pollen reveals how multiple threats could contribute to bee decline

Recent research from Centrella et al. shows the effects agriculture and associated pesticides are having on bees in terms of both their diets and the offspring they produce. Here they discuss their findings.

Threats to bee pollinators such as land use change, high pesticide use, and reduced floral diet diversity are usually assessed independently, even though we know that bees face these threats simultaneously in the agricultural systems where we rely on them for pollination. In our research, we asked how multiple threats impacted solitary mason bees across 17 apple orchards in the Fingerlakes Region of New York.

Photo 2 JAE Blog_Centrella
A mason bee nest shelter with a bee nest tube pulled forward, shown at an apple orchard site in upstate New York. These nest tubes allowed the authors to transport bee pollen and offspring to the lab for analysis. Photo: Mary Centrella

We allowed the adult females to construct nests, and then collected the pollen provisions and offspring they produced for analysis in the lab. We analysed pollen under a microscope to assess floral diversity and we used high-performance liquid chromatography (HPLC) techniques to evaluate pesticide residues in bee-collected pollen.

We allowed offspring to develop and then counted and weighed the female offspring produced to assess bee response. Finally, we use path analysis techniques to test the effects of different habitats, pollen diet diversity, and pollen pesticide exposure on female mason bee number and weight at each of our orchards.

This sped-up video shows adult female mason bees provisioning their nests in early spring. Notice the bright yellow pollen packed under bee abdomens as they return from foraging trips. Video: Bryan Danforth

Overall, our research suggests that agricultural habitats negatively impact mason bee offspring by reducing pollen diet diversity and increasing pesticide risk levels in bee-collected pollen. Our results indicate that an increasing proportion of agricultural habitats surrounding bee nests has led to reduced floral diet diversity in bee-collected pollen, which in turn resulted in fewer female offspring. Our results also suggest that, as surrounding agricultural habitats increase, bees collect a higher proportion of Rosaceae (likely apple) pollen, leading to increased fungicide risk levels, and, ultimately, reducing female offspring weight. In contrast, as proportion surrounding open habitats (wildflowers, grassland, pasture) increased, female offspring weight increased.

Photo 3 JAE Blog_Centrella
A cross-section of a mason bee nest tube reveals pollen provisions with white eggs on top of them separated by mud divisions. Larger pollen provisions towards the back of the nest (left) are likely female offspring, while smaller provisions towards the front (right) are likely male. Notice the thicker, final mud plug used to to protect the nest entrance. Photo: Bryan Danforth

Our results suggest that increasing agricultural habitats surrounding apple orchards could negatively impact solitary mason bee populations in the long term, by reducing offspring number and weight. To promote healthy mason bee populations in apple, we must maintain open habitats and floral resource diversity, while reducing fungicide exposure. Importantly, our results indicate that agricultural habitats can negatively impact mason bee populations indirectly through their influence over other, co-occurring threats. This shows the importance of accounting for multiple, co-occurring threats to bee populations when working to understand and combat pollinator decline.

Read the full article, Diet diversity and pesticide risk mediate the negative effects of land use change on solitary bee offspring production, in Journal of Applied Ecology.

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